Sven-Christian Ebenhag

Time Transfer by Passive Listening Over a 10-Gb/s Optical Fiber

A technique for time and frequency transfer over an asynchronous fiber optical transmission control protocol (TCP)/IP network is being developed in Sweden by SP Measurement Technology together with STUPI. The technique is based on passive listening to existing data traffic at 10 Gb/s in the network. Since the network is asynchronous, intermediate supporting clocks will be located and compared at each router. We detect, with a specially designed high-speed optoelectronic device, a header recognizer, the frame alignment bytes of the synchronous optical network (SONET)/synchronous digital hierarchy (SDH) protocol, as a reference for the supporting clock comparison. The goal of the project is to establish a time transfer system with an accuracy on the nanosecond level. In this paper, we present the results of a time transfer over a distance of 5 km. We have compared two clocks: a cesium clock at the Swedish National Laboratory for time and frequency and a remote rubidium clock. The results of the time transfer with the fiber link have been simultaneously compared to measurements with a Global Positioning System (GPS) carrier phase link in terms of precision and stability. The root-mean-square (rms) difference between the time difference measured with the fiber link and the GPS link is approximately 300 ps. A large part of the difference is due to the heating of the GPS antenna cable, which introduces daily delay variations on the order of 1 ns from peak to peak. For one of the days with small day-to-day variations in temperature, the corresponding rms difference is 72 ps, and the Allan deviation is below 30 ps for averaging times longer than 5 min.

Measurements and Error Sources in Time Transfer Using Asynchronous Fiber Network

We have performed time transfer experiments based on passive listening in fiber optical networks using Packet over synchronous optical networking (SONET)/synchronous digital hierarchy (SDH). The experiments have been performed with different complexity and over different distances. For assessment of the results, we have used a GPS link based on carrier-phase observations. On a 560-km link, precision that is relative to the GPS link of <; 1 ns has been obtained over several months. In this paper, we describe and quantify the different error sources influencing the fiber time transfer measurements. We show that the temperature dependence of the optical fiber is the major contribution to the error budget, and, thus, reducing this effect should be the best way of improving the results.

Time and frequency transfer in an asynchronous TCP/IP over SDH-network utilizing passive listening

A technique for time and frequency transfer over an asynchronous TCP/IP network is being developed by SP, Swedish National Testing and Research Institute together with STUPI. When implemented, users will be able to compare their clocks by connecting to the system. The technique is based on passive listening to existing data traffic in the network. Since the network is asynchronous, intermediate clocks are located and compared at each router. We use the frame alignment bytes of the SONET/SDH protocol as references in order to compare these clocks. As a test bed for the experiment, we will use the Swedish University Computer Network (SUNET). A preliminary assessment of the technique in a lab environment will be performed late 2005

Time and Frequency Transfer in Optical Fibers

The development towards more services in the digital domain, based on computers and server logs at different locations and in different networks, increases the need for high precision time indication. Even though GPS can support this with sufficient precision, many users do not have access to outdoor antennas. Furthermore, there is vulnerability in the weak radio-transmission from the satellites (NSTAC) as well as the dependence on the continuous replacement of old and outdated satellites (Chaplain). Therefore, alternative systems to support precise time are needed. Standardization of time transfer of a master clock is done for example in the IRIG system, but this one-way time transfer system do not take variations in transfer time into account, mainly because it is supposed to work on short distances (IRIG). In additional efforts to meet this request, several time and frequency transfer methods using optical fibers have been developed or are under development, using dedicated fibers (Kihara; Jefferts; Ebenhag2008; Kefelian), dedicated capacity in existing fiber networks (Calhoun) or already existing synchronization in active fiber networks (Emardson, Ebenhag2010a). A similarity of all these techniques is the need for two-way communication to compensate for the inevitable variations of propagation time, such as variation of temperature and mechanical stress along the transmission path. A two-way connection may however be undesirable when many users are connected in one network, or when user privacy is requested. As an alternative, a one-way transmission over fiber optic wavelength division multiplexing network with detection of variation in propagation time has been presented (Ebenhag2010b, Hanssen). The general conception of fiber optic communication is the transmission of digital data from one user to another, and through recovery of the phase variation of the bit-slots after reception, the exact time it has taken to transfer the data is of low importance. The individual packets of the data may even follow different paths with different propagation time, and still be interpreted correctly at the user end. Physical effects such as noise, dispersion and polarization dependence are important, but as long as each bit can be detected correctly, slow variations in propagation time do not affect the communication. When the fiber is used to transmit time or frequency however, the physical properties of the transmission link become very important. Even though time and frequency may appear as two faces of the same parameter, there are differences in the requirement of a transmission link. For time transfer, any variations in the delay through the link must be compensated for, either in a real time compensator or through post processing. For frequency transfer, the frequency shift caused by the rapidity of a change in the fiber delay must be handled.

A fiber based frequency distribution system with enchanced output phase stability

Experimental results on the stability of the output phase of a frequency distribution system from several days of measurement is presented, in addition to a discussion regarding the influence of control loop parameters. The setup handles the issue that the output phase stability of a system depends on perturbations along the transmission length. This is especially critical if the signal is transmitted through optical fiber, at lengths of a few 100 m. An experimental evaluation using a laser based transmitter at a wavelength of 850 nm, and 625 m of multimode fiber where 575 m where placed outdoor, a temperature dependence of 100 ps/°C was detected. To compensate for these slow variations in real time, a setup using two-way transmission, in conjunction with an adjustable optical delay, was constructed. This device is adjusted to induce a delay variation of equal magnitude but opposite direction, in comparison to the delay change of the fiber. Calculating the modified Allan deviation of the transmitted signal, it is apparent that without active compensation, the deviation at τ below 1000 s is comparable to the values from the measurement system without transmission. At longer integration times, however, the slow variations in the fiber transmission will deteriorate the modified ADEV substantially. When activating the dynamic adjustment of pre-delay in the system, the deviation at shorter times will increase with a few dB, however, the modified ADEV decreases continuously with τ, eventually below the values for the uncompensated system. In conclusion, activating a dynamically controlled pre-delay in a fiber based frequency transmission system will induce a small penalty on fast variations of the output phase, however giving a remarkable improvement on slower variations. The usefulness of this added functionality must therefore be determined by the application of the signal.

Fiber based one-way time transfer with enhanced accuracy

To meet the request for access to accurate and reliable time, several time and frequency transfer methods using optical fibers have been developed or are under development. These fiber based techniques will overcome the issues of vulnerability in radio- and satellite solutions; however, they all rely on two-way transmission when variations in transfer time must be compensated for. As an alternative, a one-way transmission over fiber optic WDM-network has been proposed, with estimation of variation in transfer time based on detection of transfer time difference between two co-propagating lightwaves at different wavelengths. The technique was presented previously when the two wavelengths were far apart. Here we present results from an experiment where both wavelengths are within the optical C-band, i.e. within the gain bandwidth of Erbium-doped fiber amplifiers. Thereby it is proven that the technique is usable to a larger extent than previously demonstrated.

Active detection of propagation delay variations in single way time transfer utilizing dual wavelengths in an optical fiber network

Several communication systems of today rely on the real time accessibility of accurate time and frequency measures and there is an increasing demand for the development of new and redundant methods for the distribution of these measures. The classical two-way method is able to compensate for the inevitable variations in the time and frequency propagation delay. The two-way method is used for time transfer in free space, electrical or optical domain, but has the disadvantage of often using two different paths for transmitting back and forward. The paths may be of equal length and have equal propagation delay, but nevertheless there is often a remaining asymmetry in the propagation paths. The inevitable asymmetry between the paths in the time transfer delay must be detected and compensated for, if an accuracy better than µs-level is needed for transmission distance exceeding a few km. Furthermore, if the number of users is high, there will be a complex and large network of two-way time signal transmissions. Therefore, a solution using one-way broadcasting would be more desirable, and would be possible if the variations in transmission time could be estimated from the received data at the far (user) end. The one-way method uses only one path of transmission and is possible to implement in existing Wavelength Division Multiplexing-networks. Proof of concept and results of this one-way time transfer technique based on transmission of a repetitive signal, modulated on two lasers at different wavelengths 8 nm apart and transmitted through an optical fiber, has been presented previously. These data showed a strong correlation between a change in transfer time at one wavelength, and the transfer time difference for the signals at the two wavelengths. In this paper, the setup and the measurement results have been improved and new data is collected which shows improvement in the reliability and quality of this technique. The stability is improved through component analysis and minimizing error sources. The distance is improved from 38km to 160km.

Time transfer using an asynchronous computer network: Results from three weeks of measurements

We have performed a time transfer experiment between two atomic clocks, over a distance of approximately 75 km using an 10 Gbit/s asynchronous fiber-optic computer network. The time transfer was accomplished through passive listening on existing data traffic and a pilot sequence in the SDH bit stream. In order to assess the fiber-link clock comparison, we simultaneously compared the clocks using a GPS carrier phase link. The standard deviation of the difference between the two time transfer links over the three-week time period was 243 ps.

Time transfer using an asynchronous computer network: An analysis of error sources

We have performed a time transfer over a distance of approximately 75 km using an asynchronous computer network based on optical fibers. In order to validate the results from this fiber-link, we have compared the results with a GPS-link, which consists of carrier phase observations. All electronic cabinets were equipped with temperature and humidity sensors. Here we present experiments where the temperature and humidity of the delay in the electrical components were investigated. All components showed some temperature dependence, but no significant humidity dependence was found. By using the derived temperature coefficient for the components the standard deviation of the difference between the fiber link and GPS link decreased from 243 ps to 184 ps.

Real-time phase stable one-way frequency transfer over optical fiber

The fundamental and most straightforward method for high performance time and frequency transfer is the two-way technique, which is suitable when the user has access to the whole system, and when both transmission paths are equal or with a known and predictable asymmetry. Furthermore it is most practical when the numbers of users are limited and when no security concerns limit the bidirectional connectivity.